recipes:busybox: Enable nice for builds without GPLv3 packages

The nice command from coreutils is released under GPLv3 and therefore
missing from No-GPLv3 builds. Enable the busybox nice applet for these
cases.

SCH-41

recipes-core/busybox/busybox_%.bbappend

+FILESEXTRAPATHS:prepend := "${THISDIR}/files:"
+SRC_URI:append:nogplv3 = " file://fragment.cfg"

recipes-core/busybox/files/fragment.cfg

+CONFIG_NICE=y